1. Field of the Invention
This invention relates to inverter chargers.
2. Description of the Related Art
Inverter chargers are generally well known and are used in the event of a utility supply failure, to provide continuity of supply for a load from a battery, and/or to provide a supplementary supply to cope with peak power requirements which exceed the capability of a supply generator, power from the utility or from the generator, as the case may be, being used to charge the battery. Inverter chargers may also be arranged to feed surplus energy back to the utility.
Inverter chargers comprise an inverter unit, which serves to provide a.c. for the load by converting d.c. from the battery, and control/switching apparatus for control of the supplementary/surplus power supply as required, and for providing current for charging the battery from the utility or generator.
The provision of a sinusoidal output voltage, similar to the waveform of an a.c. 50 hertz or 60 hertz utility supply, is highly desirable, since some loads, such as a.c. motors for example, operate inefficiently when fed from an a.c. source having a high harmonic content such as a square wave source. Also, provision for drawing a sinusoidal current from a 50 hertz or 60 hertz utility supply or generator is highly desirable for maximum utilization of generator power rating and most efficient usage of utility power when drawing power from the utility supply or generator to charge the battery, or when delivering surplus power from the inverter to the utility or when delivering supplementary power in support of the generator.
It will thus be appreciated that an inverter charger may be used as a part of standby power or emergency power supply apparatus wherein power for a load is provided via the inverter charger by the battery when the utility supply or generator, as the case may be, fails and wherein the battery is charged via the inverter charger from the generator or utility supply, during periods when it is available, and wherein some supplementary power for the load from the battery may also be provided in order satisfy peak load demand when this exceeds the capability of a generator having insufficient capacity to meet such demands.
Known inverter chargers capable of providing the functions as just before described, and having a sinusoidal output, tend to be expensive to fabricate and somewhat inefficient in operation due to the design of the inverter unit. In order to produce an inexpensive inverter charger, it is desirable to use one inverter both for battery charging purposes and to supply a load but with such arrangements regulation for both functions poses problems.
It is an important object of the present invention to provide an inverter charger wherein the shortcomings of such known apparatus are obviated, at least in part, and wherein regulation of both charging and load supply functions is facilitated.
Since the present invention is concerned more particularly with the provision of an inverter charger, control/switching apparatus as used in inverter chargers, and which may take any suitable form, will not be described herein in detail.
According to the present invention, an inverter charger comprises a single phase inverter unit having eight inverter stages arranged for connection to a common D.C source, which stages provide similar stepped alternating waveforms of substantially the same amplitude and waveshape, eight individual similar single phase two winding transformers, one primary winding provided on each of the said single phase transformers to which, respective ones of the said inverter stages are connected, one secondary winding provided on each of the said single phase transformers, two secondary winding sets, each of which sets comprises four of the said secondary windings connected in series, the secondary windings of each set being driven sequentially to produce in combination, a generally sinusoidal step function waveform comprising fifteen degree incremental steps, the two step function waveforms thus produced, one by each set, being serially combined, and mutually relatively phase controlled to produce a single phase sinusoidal output voltage for a load normally fed from a utility or generator supply so as to provide continuity of supply if the utility or generator supply fails, output voltage regulator means operative to regulate the said output voltage in dependence upon the relative phase between the said two step function waveforms, a battery for supplying d.c. to the said inverter unit, control means for controlling the supply of charging current fed from the utility or generator supply to the said battery and for controlling provision by the inverter of supplementary power when the capacity of the generator is inadequate, and switch means responsive to a control signal produced by the control means in dependence upon battery voltage to connect the inverter to the utility or generator so that controlled sinusoidal charging current is fed from the utility or generator back through the inverter to charge the battery, and wherein each of the said stages comprise, four transistor switches connected in bridge configuration across the said primary winding of one of the said transformers with which they are operatively associated, conductivity of the said transistor switches being controlled so as to produce dwell periods in the output voltage from each stage, whereby provision by the inverter of charging current in one direction or supplementary power in the other direction are both controllable.
It will be readily apparent that the relative phase of the two step-function waveforms, can be controlled manually, or automatically by appropriate operation of the drive circuit, thereby to effect the regulation.
By utilizing eight transformers in two sets of four, a regulated sinusoidal output voltage is provided and sinusoidal current is drawn from the utility or generator for regulated charging, efficiently and relatively economically as will hereinafter be explained. One embodiment of the invention will now be described with reference to the accompanying drawings.